The invention relates to central solar receivers with volumetric solar absorber.
A central solar receiver absorbs concentrated sunlight at a high temperature, commonly about 700.degree.-1500.degree. C. and transfers the heat generated by the solar absorber therein to a working fluid which either serves as heat carrier fluid or else is designed to perform a thermochemical process. In one known kind of central solar receiver, a so-called tubular receiver, the working fluid flows inside tubes located usually near the inner periphery of the solar receiver housing. In such receiver, solar radiation is absorbed at the outer surface of the said tubes and is transmitted as heat to the working fluid therein which is thus heated up. The overall resistance to heat transfer and the ensuing heat loss in such tubular central solar receivers is relatively high.
In view of this deficiency of the tubular central solar receivers it has already been proposed to design central solar receivers with volumetric solar absorbers and the feasibility of this concept was demonstrated, inter alia, in three separate papers by H. W. Fricker et al., R. Buck and W. Pritzkow in "Solar Thermal Technology-Research Development and Application", Proceedings of the Fourth International Symposium, Sante Fe, NM, USA, June 1988, Hemisphere Publishing Corp., New York, pages 265-277, 279-286 and 635-643 respectively. These studies show that with a volumetric solar absorber-type central solar receiver it is possible to handle solar fluxes about 5-10 times larger than with existing tubular receivers and that in consequence the receiver size and weight can be reduced. It was also shown that in volumetric solar absorbers the temperature difference between the absorber and the working fluid is relatively small which enables the lowering of the average absorber temperature whereby radiation losses are reduced and material constraints are somewhat relieved. Furthermore, it was shown that the start-up time and system response to sunlight fluctuations are relatively fast whereby the efficiency of the system is increased.
Known volumetric solar absorbers are constructs in the form of foam, honeycomb or wire mesh matrices and they are made of materials capable of withstanding high temperatures, e.g. ceramics or special metal alloys such as, for example, stainless steel, and the flow of the working fluid in such known receivers is essentially either codirectional with or counter-directional to the incident solar radiation.
Investigations conducted in accordance with the present invention revealed that central solar receivers with prior art volumetric solar absorbers have a number of deficiencies which may be summed up as follows:
(1) Sunlight cannot penetrate deep into the absorbers and most of the radiation is absorbed at or near the front surface thereof. PA0 (2) The transfer of energy between elements of the absorber matrix by conduction and reradiation is very limited. Accordingly, the matrix cannot redistribute the energy it absorbs and is incapable of adjusting to the normally very non-uniform incoming radiation it receives from the concentrators. Consequently, large temperature gradients and high local temperatures develop over the matrix. PA0 (3) The working fluid flow cannot be adjusted prior to or during operation in a manner which would match the incoming radiation flux and even out the absorber temperature distribution. PA0 (4) In cases where the absorber surface acts as catalyst for a heat induced reaction in a gas mixture, the contact time between the gas and the surface in the region exposed to direct sunlight is relatively short. Therefore, some of the reaction occurs under less favourable conditions, producing an overall lower yield. PA0 (5) Temperature measurements over and within the absorber bed are difficult to perform. PA0 (6) Damages to the absorber such as surface oxidation, rupture in case of wire mesh absorber construct and cracking in case of a foam or honeycomb type absorber construct, occur within several days of operation. PA0 (7) The reradiation energy losses are higher than expected due to the existence of local temperature peaks which may exceed the average absorber temperature by several hundred degrees.
It is the object of the present invention to provide an improved volumetric solar absorber for a central solar receiver.